doc: dynamic mount requires dynroot and fakestat

Document that dynamic mount (/afs/.:mount) requires
dynroot and fakestat on non-linux unix.

Reviewed-on: http://gerrit.openafs.org/8739
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
(cherry picked from commit 7dbb35a89f42743ff0a29bc9822b94141a2a8323)

Change-Id: Ia77835cd1830c7bc5dde573f2e890341aa06bc21
Reviewed-on: http://gerrit.openafs.org/9519
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>